How to plot a piecewise function?

[Roger Stafford]

The problem with your code is that when you apply an ‘if’ to an entire vector, as you have done with “if x<0”, it is not regarded as true by matlab unless all elements of that vector are true, and that of course is not the case, so the ‘if’ will fail. The same hold true with your other two if’s - they will all fail and y is never set.

[Stephen23]

Original question asked by Alexandros Dhimas on 20th December 2016 retrieved from Google Cache:

http://web.archive.org/web/20201225181844/https://webcache.googleusercontent.com/search?q=cache%3ASu-ZTnJ4_rMJ%3Ahttps%3A%2F%2Fwww.mathworks.com%2Fmatlabcentral%2Fanswers%2F317651-how-to-plot-a-polyclonal-expression-using-if-statement

How to plot a polyclonal expression using if statement?

Hello, I' ve got a polyclonal function to plot (using if statement) as shown on this picture:

for -5<x<30.

My thougth on the solution was:

x=linspace(-5,30,1000);
if x<0
      y=10*x.^0;
  else if x>=0 & x<9
          y=10*x+10;
      else if 9<=x 
              y=15*sqrt(4*x)+10;
      end
      end
  end
  plot(x,y)

But i get this on the command window "Error using plot A numeric or double convertible argument is expected"

I can't find any mistakes on my code.

[Walter Roberson]

This is not a Duplicate.

Answer 1

[Alexandros Dhimas]

Solution usin if statement:

x=linspace(-5,30,1000);

for i=1:1000

if x(i)<0
     y(i)=10;

elseif x(i)>=0 & x(i)<9

y(i)=10*x(i)+10;

   else 

y(i)=15*sqrt(4*x(i))+10;

end
end

plot(x,y)

Answer 2

[Walter Roberson]

Either use a for loop or (better) use logical indexing. http://blogs.mathworks.com/steve/2008/01/28/logical-indexing/

Answer 3

[Roger Stafford]

If you only want to plot the function, do this:

x1 = linspace(-5,0,50); y1 = 10*ones(1,50);
x2 = linspace(0,9,90);  y2 = 10*x2+10;
x3 = linspace(9,30,210); y3 = 15*sqrt(4*x3)+10;
plot([x1,x2,x3],[y1,y2,y3])

Another way:

x = linspace(-5,30,400);
y = (x<0)*10+(x>=0&x<9).*(10*x+10)+(x>=9).*(15*sqrt(4*abs(x))+10);
plot(x,y)